1998 Other Toyota Models Decreased fuel mileage

My '98 toyota Tacoma 4X4 has started consuming twice it's usual gas this summer yet it runs fine. I changed the fuel & air filters, spark plugs, spark plug wires. The garage says my oxygen sensors are fine. Toyota can't analyse it since no warning light comes on. They said that changing the timing belt MIGHT do something but that's a lot of work and $ to do on spec.

Any suggestions as to what I should try next? It's using almost double the amount of fuel it uses in winter.

Thanks